Seville faces being added to a Unesco blacklist as building work on a 40-storey skyscraper begins to change the southern Spanish city's skyline.

The half-built Pelli tower is casting a growing shadow across one of the country's most-visited cities and over a cluster of 13th-century buildings which have been designated a world heritage site by Unesco.

In a report leaked to local newspapers, Unesco experts denounced the "substantial" impact on several historic buildings. "It is surrounded by historic conservation areas," it said. "There is an excessive and undoubtedly negative impact."

Among the buildings affected are the cathedral, the Alcázar, the Giralda minaretand the Archive of the Indies, which together make up the world heritage site.

"They form a remarkable monumental complex in the heart of Seville," Unesco said. "The Giralda minaret is the masterpiece of Almohad architecture. This will end the Giralda tower's unrivalled pre-eminence in the urban landscape."

At a meeting in June Unesco must decide what to do about the city's refusal to halt construction. Among the options is to place it on the "in danger" list, or to strike Seville off the list of world heritage sites.

The "in danger" list includes sites such as Bamiyan valley in Afghanistan and the earthquake-ravaged city of Bam in Iran.

The Pelli tower is being built on the site of the 1992 Expo across Guadalquivir river from Seville's historic city centre.

The 12th-century riverside Tower of Gold is another monument that will be dwarfed by the 178-metre (580ft) Pelli building.

Unesco has asked local authorities to at least reduce the height of the building, but pleas made over the past four years have been ignored. The organisation said it had asked the city "to halt the construction works and reconsider the current high-rise project".

"Attempts are made to offer help to places so that they can solve problems," one expert on world heritage sites said. "But there comes a time when there is no hope for that."

That time appears to have come for Seville, where the Pelli tower has already reached 12 storeys. "They are meant to be adding another floor each week," said the expert.

El País newspaper said the report had been leaked by city hall authorities, which it read as a sign that the new city council, which was elected last year, might try to lower the height of the building.